Fix regression in calling core only plugins from extensions (#5875)
Fix regression in calling core only plugins from extensions (#5875)

PR osquery/osquery#5464 makes it impossible to call plugins
which are only registered in the core, as its happening with osquery/osquery#5839.

What happens when this is working is that the filesystem plugin is not present
in the extension registry, so when Registry::call tries to search for a plugin
that provides the "config" feature, it won't find one and the resulting plugin name,
and later item_name, will be empty.

This results in the RegistryInterface::call reroute the call to the core,
through Thrift, which will end up in the plugin correctly answering.
hackgnar/osquery,hackgnar/osquery,hackgnar/osquery